Hydrazine and related compounds are used as fuels for space launch vehicles. Hypergolic rocket propellants (hydrazines and nitrogen tetroxide) used in both Air Force and civilian rocket launch operations are extremely hazardous materials whose atmospheric release could present a serious threat to health and the environment. Hydrazine and its methyl-substituted derivatives [methylhydrazine (monomethylhydrazine) and 1,1-dimethylhydrazine (unsymmetrical dimethylhydrazine)], referred to herein as hydrazine-fuels, are flammable toxic substances and suspected carcinogens with current threshold limit values of one hundred to five hundred parts per billion (ppb). The American Conference of Government Industrial Hygienists (ACGIH) has recommended that these exposure limits be lowered to ten ppb for all hydrazines. Similarly, nitrogen dioxide, the spontaneous decomposition product of the oxidizer nitrogen tetroxide, is a hazardous material with a threshold limit value of three ppm. It is imperative that accidental vapor and liquid releases of these materials be rapidly identified and located to minimize exposure of military and civilian personnel and native wildlife to propellant vapor, and to facilitate remedial cleanup operations. Hence, hydrazine fuel vapors are highly toxic, with maximum exposure levels, averaged over eight hours, likely to be set as low as ten parts per billion. Regulatory compliance requires the detection of hydrazine fuel vapors with enough sensitivity to trigger alarms before those exposure levels are reached. Possible sources of hydrazine fuel vapor releases are widely dispersed through launch facilities, which typically span large land areas.
Monitoring of hydrazine fuel vapors is currently accomplished by fixed or portable instruments based on electrochemical sensors or colorimetric chemical indicators. Electrochemical hydrazine-fuel monitors measure electric current produced when a hydrazine-fuel is oxidized in an electrochemical cell, but such monitors are not suitable for monitoring gaseous hydrazine-fuels over wide areas.
Colorimetric monitors use moving paper tapes impregnated with reagents which change color in the presence of hydrazine fuel vapors. Changes in color are automatically detected by photometry. Hydrazine-fuel area monitors using colorimetry have been made by MDA Scientific, Inc. and GMD Systems, Inc. The colorimetric hydrazine-fuel monitors use paper tape impregnated with phosphomolybdic acid (PMA) which moves at a constant rate past an air intake. When exposed to a hydrazine-fuel vapor, the tape changes color. The change in color is detected by comparing light reflected by the tape prior to and after exposure to the air intake. These devices are large, expensive, contain moving parts, require frequent maintenance and are not well suited for systems-oriented operations. Monitors using these sensors are cumbersome, require high maintenance, are not well suited for centralized network operation over a large area and are unable to detect hydrazine fuel vapors below twenty parts per billion.
Another type of colorimetric monitor are the paper cardboard badges which have been used as personal hydrazine-fuel monitors for launch personnel wearing the badges about launch sites. The colorimetric dosimeter badges are used for additional protection. These personnel badges use paper impregnated with reagents. Changes in color indicating exposure to hydrazine fuels are detected visually. The cardboard badges contain pieces of paper impregnated with colorimetric hydrazine-fuel indicators. Usually, more than one indicator is used to provide assurance against false positives. Vanillin, dinitrobenzaldehyde and para-dimethylaminobenzaldehyde are generally used as indicators. The hydrazine-fuel exposure dose is estimated by visually comparing the indicator-impregnated pieces of paper with a color chart. Badges provide only an after-exposure indication of hydrazine-fuel exposure, and in most cases will not help in locating the source of hydrazine fuel vapor leaks.
Fiber optic-based personal dosimeters have been developed. Geo-Centers, Inc. developed a personal hydrazine dosimeter based on an optical fiber chemical optrode. The dosimeter consists of a short length of porous optical fiber impregnated with the colorimetric reagent vanillin. When exposed to hydrazine, vanillin turns yellow by strongly absorbing blue light. The dosimeter operates by absorption with a blue light source, for example, blue light-emitting diode (LED) at one end and a photodetector at the other. Exposure of the fiber to hydrazine causes the transmission of light within the fiber to decrease sharply, and thus reduces the photodetector output signal. This device uses the length of a modified optical fiber incorporating a colorimetric hydrazine indicator as the hydrazine sensor. A disadvantage of this device is that it can not be used to accurately locate a poisonous hydrazine cloud. This device operates by monitoring the light transmitted through the fiber, rather than the backreflected light at a well defined point. Thus, this device is basically a single-sensor device, not compatible with multiple-point detection by laser interrogation. In particular, it can not be interrogated using the Optical Time Domain Reflectometry (OTDR) techniques for multiple-point detection for determining the exact location and extent of a hydrazine-fuel release. The choice of vanillin as a hydrazine indicator dictates that the sensor must be interrogated with blue light, instead of visible red or near-infrared light. The limited use of only blue light has two additional disadvantages which prevent the use of the Geo-Centers sensor in a multiple-point hydrazine-fuel-monitoring network. Firstly, sources of blue light are blue LEDs which typically do not produce sufficient light intensity to interrogate multiple sensors. Secondly, blue light is not transmitted very well by optical fiber which limits severely how far a sensing element can be from the light source and the photodetector. The use of vanillin hydrazine sensors is unsuitable in a multiple point hydrazine-fuel-monitoring network where high intensity illuminating laser light is required to propagate over the wide area.
United States Statutory Invention Registration #H1297, "Detection Device for Hazardous Materials", J. K. Partin and A. Grey, Issued Apr. 5, 1990, describes a fiber optic dosimeter suitable for the detection of hydrazine vapors. It uses evanescent wave absorption in an optical fiber, and uses colorimetric indicator techniques. The reagent used to detect the presence of hydrazine is nitrobenzaldehyde, which is not suitable for interrogation by diode lasers, because after hydrazine exposure, it absorbs light in the blue region of the spectrum, rather than in the visible red or near infra-red. A tungsten/halogen lamp is used but it cannot be pigtailed to an optical fiber and requires the use of an alignment micrometer stage and a focusing lens. The dosimeter detects the presence of hydrazine by absorption, and not by reflection. Thus, this dosimeter cannot be used with optical time domain reflectometry techniques to interrogate the sensor, and the device is only capable of sensing hydrazine at a single point. This dosimeter is not suitable for use in a multipoint fiber optic sensor network. Also, in order to obtain a reference signal measuring how bright the lamp is, the dosimeter detects the red component of the lamp light which propagates through the fiber essentially unattenuated, whether hydrazine is present or not. In order to do so, the dosimeter needs a somewhat complicated detector involving a beam splitter, two interference filters (one for blue light and one for red light), two photodiodes and a second alignment micrometer stage.
U.S. Pat. No. 5,059,790 "Reservoir Fiber Optic Chemical Sensors", Klainer et al., Issued Oct. 22 1991, and U.S. Pat. No. 5,116,759, "Reservoir Chemical Sensors", Klainer et al., Issued May 26 1992, describe a general class of chemical sensors interrogated by optical fibers in which the indicator chemistry takes place in the liquid phase within a special reservoir. The chemical species being sensed enters through a specialized permeable membrane. The selected membrane is for sensing a number of chemical species, amongst which is hydrazine. These systems use a colorimetric indicator general technique. The reagent employed for sensing hydrazine is a cupric neocuproine solution which, upon exposure to hydrazine, absorbs light in the blue region of the spectrum. Optical fibers are only used to convey light in and out of the sensing reservoir. These systems are not suitable for wide area hydrazine detection using a network of distributed sensors adapted for use with conventional visible red and near infra-red lasers.
Evanescent wave fiber optic sensors are well known. Sol-gel glass techniques have been used to make evanescent wave fiber optic chemical sensors. Sol-gel porous glass sensors have been used in medical applications to detect various gases in and the pH of circulating blood. A catheterized optical fiber with a single distal end sensor has been used. These sensors typically use short wavelength blue light lamps for real time interrogation. These sensors have not been adapted to wide area detection of hydrazine-fuels using interrogating lasers.
Most available hydrazine-fuel indicators display changes in the blue wavelength range, for which no convenient laser sources exist, and which is strongly absorbed in optical fibers, rendering hydrazine-fuel optical detection unsuitable for low cost conventional visible red and near infra-red lasers. These hydrazine-fuel detectors are not adaptable to fiber optic networks for detecting hydrazine-fuels over a wide area. Further, previously used colorimetric hydrazine-fuel sensors are irreversible in that once the sensors become exposed to a hydrazine fuel, the sensor permanently remains in an exposure-indicating colorimetric state. Hence, the irreversible colorimetric hydrazine fuel sensor does not indicate the proximal time of exposure and the current hydrazine-fuel concentration, and may no longer be used as an effective sensor after an initial exposure. These and other disadvantages are solved or reduced using the present invention.